The electrospray process consists of flowing sample liquid through a small tube or capillary which is maintained at a high voltage with respect to a nearby surface. The liquid is dispersed into fine electrically charged droplets or by the voltage gradient of the tip of the capillary. The ionization mechanism involves the desorption at atmospheric pressure of ions from the fine electrically charged droplets. In many cases a heated gas is flowed in counter-current to the electrospray to enhance dissolution of the electrospray droplets. The ions created by the electrospray are then mass analyzed in a mass analyzer such as a mass spectrometer.
Under the appropriate conditions, the electrospray resembles a symmetrical cone consisting of a very fine mist (or fog) of droplets (ca. 1 .mu.m in diameter). Excellent sensitivity and ion current stability can be obtained if the fine mist is produced. Unfortunately, the electrospray "quality" is highly dependent on the bulk properties of the solution being analyzed. The most important of which are surface tension and conductivity. A poor quality electrospray may contain larger droplets (&gt;10 .mu.m diameter) or a non-dispersed droplet stream.
The use of sheath liquid and a focusing gas are often used to insure stable sprays when electrospraying high aqueous content sample solutions. U.S. Pat. Nos. 4,977,785; 4,885,076, and copending patent application Ser. No. 575,183 describe electrospray ion sources including a sheathing liquid. The system described in said copending application uses a multilayer flow needle assembly which provides stable ion currents over a wide range of solution conditions (e.g., pH, conductivity, and aqueous content). The multilayer flow needle consists of a central tube for sample introduction, a second concentric tube for sheath liquid introduction, and a third concentric tube for gas flow addition. The primary purpose of the liquid sheath is to reduce the surface tension of the eluent stream in order to allow electrospray compatibility with solutions of high aqueous content. The concentric gas flow is important in that it improves ion current stability and sensitivity when solutions of high conductivity are electrosprayed. The sheath flow can result in the production of undesirable neutral noise in the mass spectrometer.
The neutrals are apparently caused by droplets which are directed through the ESI atmosphere/vacuum interface by the sheath gas flow. It is believed that the droplets cool as they enter vacuum and eventually freeze, forming tiny ice pellets. These particles are accelerated by the free jet expansion to supersonic velocities. In the free jet expansion, the particles attain enough kinetic energy to produce secondary ions when they impinge upon a surface. This results in intense noise "spikes" at the detector of the mass spectrometer.